Simultaneous, spatially resolved measurements of mixture fraction and absolute hydroxyl radical concentration are obtained for the first time in nonpremixed, turbulent, hydrogenair flames. This is accomplished by combining spontaneous Raman scattering with linear, laser-induced fluorescence (LIF). The Raman scattering data define the instantaneous, local collisional quenching environment of the OH molecules, allowing quenching corrections to be applied for each laser shot and making the linear LIF measurements quantitative. The effect of Damköhler number on OH superequilibrium is determined by performing measurements at selected locations in two argon-diluted hydrogen flames (Reynolds numbers 8,500 and 17,000). Results demonstrate that departures from chemical equilibrium in these flames are a consequence of the fact that time scales for turbulent transport are competitive with time scales for three-body radical recombination reactions. Due to the slow characteristic time for the radical recombination, convective histories, as well as instantaneous local conditions, determine hydroxyl concentrations. Damköhler numbers are not sufficiently low for the rapid bimolecular reactions to be strongly affected. Comparison of turbulent flame data with results from strained laminar flame calculation incicates that partial equilibrium of the bimolecular reactions is a good approximation near the stoichiometric mixture fraction. Comparisons of the experimental data with predictions by Monte Carlo simulations using a partial equilibrium chemistry model show good overall agreement. However, simulations predict a smaller variance of OH concentration than is measured for a given value of mixture fraction, lower OH concentrations at off-stoichiometric conditions, and a more rapid decay toward equilibrium with streamwise distance.
ASJC Scopus subject areas
- General Chemistry
- General Chemical Engineering
- Fuel Technology
- Energy Engineering and Power Technology
- General Physics and Astronomy